1. Field of the Invention
The present invention relates to an energization control apparatus for controlling the supply of electric current to a controlled component for a vehicle (hereinafter referred to as a “controlled vehicle component”) such as a glow plug.
2. Background of the Invention
Conventionally, various energization control apparatuses have been used to control the supply of electric current to controlled vehicle components, such as glow plugs used for diesel engines and heaters for heating various sensors (for example, an oxygen sensor, an NOX sensor, etc.) mounted on vehicles. A known energization control apparatus includes switching means (for example, an FET, etc.) for opening and closing a path through which electric current is supplied from a battery to a controlled vehicle component, and a computation device for turning the switching means on and off. Also, in general, such an energization control apparatus includes a temperature-sensitive element (for example, a thermistor, etc.) for protecting the switching element, such as an FET, from overheating.
On the other hand, for accurate detection of a heat-generated state by a temperature-sensitive element, the temperature-sensitive element must operate normally. Therefore, a method for detecting a failure in operation of a temperature-sensitive element has been proposed (see, for example, Patent Document 1). In the known method, a plurality of temperature-sensitive elements are provided, and, at the time of startup of a vehicle, the respective temperatures detected by the temperature-sensitive elements are compared with the ambient temperature. When the difference between the temperature detected by a certain temperature-sensitive element and the ambient temperature is greater than the differences between the temperatures detected by the other temperature-sensitive elements and the ambient temperature, a determination is made that the subject temperature-sensitive element has failed. This method makes it possible to detect not only wire-breakage, open failure, and short-circuit of a temperature-sensitive element, but also an anomalous state in which the detected temperature shifts to the high-temperature side or the low-temperature side due to deterioration of the temperature-sensitive element or other causes (high-temperature-side-shift anomaly or low-temperature-side-shift anomaly).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-211714
3. Problems to be Solved by the Invention
Ideally, electronic components which constitute an energization control apparatus, a harness connected to the energization control apparatus, and a controlled component such as a glow plug are fabricated with the intent that they do not exhibit any variance. However, since these are industrial products, in actuality, they do have tolerances; for example, several percent on plus and minus sides in relation to a center value, or several percent on the plus or minus side only (for example, on the minus side only (minus variation)). Here, an example case here will be considered where the switching means is an FET, and the controlled vehicle component is a glow plug. In such an example case, the amount of heat generated by the FET as a result of supply of electric current to the controlled vehicle component (glow plug) is affected by the resistance of the glow plug. For example, by comparing the case where a glow plug whose resistance is equal to the upper limit of the tolerance (allowable range for use) is connected to an energization control apparatus and the case where a glow plug whose resistance is equal to the lower limit of the tolerance is connected to the energization control apparatus, the FET is found to generate a larger amount of heat in the case where the glow plug whose resistance is equal to the upper limit of the tolerance is connected to the energization control apparatus.
Further, a detected temperature may greatly vary according to a position of a temperature-sensitive element, depending on whether it is disposed near the switching means or disposed at a location separated from the switching means. Due to a difference in the structure of the controlled vehicle component and a difference in the position of the temperature-sensitive element, the method described in Patent Document 1 may erroneously determine that a temperature-sensitive element is defective.
Moreover, when the above-described method is employed, at least two temperature-sensitive elements must be provided, which results in an increase in production cost.